1. Field of the Invention
The present invention relates to a continuous casting apparatus and continuous casting method for producing aluminum alloy cast bars by supplying molten alloy from a molten metal-receiving portion to a mold through a melt passage which penetrates insulation members provided between the molten metal-receiving portion and the mold; and to aluminum alloy cast bars.
2. Description of the Prior Art
In recent transportation equipment, due to desirability of reducing weight, aluminum alloy parts have come to be employed more frequently. Aluminum alloy parts for such purposes are produced by cutting an aluminum alloy bar to predetermined lengths to thereby produce raw materials for forging and forging the materials into specific parts. In this case, the aluminum alloy bar is manufactured through plastic processing and thermal processing of a material produced, for example, by horizontal continuous casting.
Generally speaking, horizontal continuous casting transforms molten metal into elongated cast ingots of, for example, round columnar, square columnar or hollow cylindrical shape, through the following steps. That is to say, molten metal which is supplied to a tundish that receives molten metal passes through a passage surrounded by a refractory material and enters an approximately horizontal cylindrical mold, where the molten metal is forcibly cooled to form a solidifying shell outside the molten metal body. When the thus produced cast ingot exits the mold, a coolant such as water is directly injected, allowing solidification of the metal to progress towards the core of the ingot to thereby attain continuous casting.
In the horizontal continuous casting, a lubricant is introduced to the inner wall surfaces of the mold on its inlet side to thereby prevent seizure of molten metal on the mold lining. In the mold, due to the difference in the gravimetric force applied to the top and bottom faces of an ingot, the lubricant climbs up from the lower part of the wall surface toward the upper part thereof. Gases produced from decomposition of the heated lubricant also move upward along the wall surface. These phenomena create unevenness between high and low portions of the mold in terms of the lubrication state between the mold inner wall and the molten metal or the solidifying shell of a cast ingot.
For example, in a lower portion of the mold, since no lubricant is present between the mold inner wall and the molten metal or the solidifying shell, the molten metal seizes on the mold's inner wall, breaking the solidifying shell to allow the not-yet-solidified molten metal to outflow, producing a large casting defect, or in an extreme case, tearing off the ingot and preventing continuation of the casting operation. Meanwhile, at an upper portion of the mold inner wall, since lubricant is present in an excessive amount, which prevents close contact between the molten metal and the mold inner wall, the molten metal cannot be sufficiently cooled by the mold, permitting blowing out of unsolidified molten metal from the upper portion of the cast ingot.
In order to overcome such essential problems involved in horizontal continuous casting of metal, a variety of countermeasures have been proposed in, for example, JP-B HEI 8-32356(hereinafter referred to as Patent Document 1), JP-A HEI 11-170009 (hereinafter referred to as Patent Document 2) and JP-B HEI 11-170014 (hereinafter referred to as Patent Document 3).
Of the above-listed Patent Documents 1, 2 and 3, Patent Documents 1 and 2 are concerned with supply of lubricant, and Patent Document 3 is directed to means for attaining uniformity in temperature distribution of the molten metal within the mold.
Patent Document 1 attempts to provide a horizontal continuous metal casting method and a relevant apparatus which are free from the problems involved with conventional horizontal continuous metal casting methods, such as the imbalance in cooling of molten metal within the mold and uneven thickness of the lubricant film on the mold inner wall, and which are capable of consistently producing high-quality cast ingots exhibiting a uniform microstructure of cast ingot and having no casting surface flaw or breakout. Specifically, this document discloses a horizontal continuous metal casting method in which, while a lubricating fluid is supplied to a forcibly cooled, virtually horizontal cylindrical mold, molten metal is supplied at the upstream end of the cylindrical mold to thereby form a columnar molten metal body, and at the downstream end of the cylindrical mold, a solidified columnar cast ingot, which has been formed as a result of solidification of the columnar molten metal body, is withdrawn, wherein the lubricating fluid is caused to permeate into the pores of the mold's permeable porous member provided on the inner wall of the cylindrical mold to thereby cause continuous seepage of the lubricating fluid onto the inner wall of the cylindrical mold that faces not-yet-solidified molten metal or now-solidifying molten metal, while the lubricating fluid and/or a gas primarily containing gas components produced from decomposition of the lubricating fluid is/are released from an ingot outlet end of the mold via grooves formed on the inner wall of the cylindrical mold, such that the amount of the lubricating fluid that seeps onto an upper portion of the mold's permeable porous member is regulated to be smaller than the amount of the lubricating fluid that seeps onto a lower portion of the mold's permeable porous member.
Patent Document 2 discloses a horizontal continuous casting method for aluminum or aluminum alloy, in which an appropriate amount of a lubricant is caused to be present uniformly on the mold's inner wall in all radial directions to thereby improve the surface quality of cast ingots, and also to enhance yield by reducing the thickness of the inverse segregation layer and thus the amount of peeling. Specifically, to attain this, a plurality of lubricant supply holes are provided at the inner wall of the upper half section of the mold, and the supply amount of the lubricant is regulated to fall within a range of 0.001 to 0.012 cc/min·mm per unit outer peripheral length of the cast ingot. Moreover, a self-lubricating carbon sleeve is shrink-fitted on the inner wall of the metallic mold to be cooled.
Patent Document 3 discloses a horizontal continuous casting apparatus having, in a gate insulating member of a mold for the apparatus, a molten metal supply inlet through which molten metal is supplied from a furnace to the mold, which is provided at a point that falls within a region extending downward from the center of the mold as viewed in its cross section, and which has a cross sectional area of 10 to 25% the entire cross section of the mold to thereby attain uniformity in temperature distribution of molten metal within the mold, to diminish the cold shut which may be formed in a lower portion of a cast ingot and to reduce the thickness of an inverse segregation layer formed in the ingot surface, and as a result, to improve yield by reducing the peeling amount of a cast ingot and simultaneously to suppress occurrence of breakout.
In recent years, in order to ensure stable production operation of horizontal continuous casting, a large amount of lubricant is often required for attaining adequate lubrication. For example, amid mounting demands for aluminum alloy parts, improvement in productivity of the raw material; i.e., aluminum alloy bars, has become of keen interest. To attain this, casting speed must be increased, which in turn requires an increase in the supply amount of lubricant for preventing seizure.
If a large amount of lubricant is supplied, however, an excessive amount of gas may be produced to cause breakout, or when an excessive amount of lubricant contacts molten metal, lubricant reaction products will be produced. These incidents are unfavorable, as they results in production of defective cast ingots.
In view of the foregoing, the present invention is directed to providing a horizontal continuous casting apparatus and a horizontal continuous casting method which enable stable and smooth high-speed casting with a reduced amount of a lubricant and which prevent occurrence of breakout and production of lubricant reaction products, attaining reduction in ingot failure, as well as an aluminum alloy cast bar produced through use of the apparatus or the method.
To attain the above object, the present invention discloses a continuous casting apparatus, a continuous casting method and an aluminum alloy cast bar having the following characteristic features.